1. Field of the Invention
The present invention relates to a connector constructed of a steel pipe which is used in a sleeper of a mixed rigid connection structure having a ladder-like shape (hereinafter, referred to as a ladder-type sleeper for railway tracks) that is integrated with longitudinal beams made of concrete and connectors constructed of steel pipes as a railway track laid on a bed of a railroad.
2. Description of Related Art
A railroad is an exclusive road constructed of a track and a base supporting the track. A railway track includes a bed, sleepers and rails and the base includes a road base supporting the bed and a railway structure. A ballast is generally used for the bed and impact force or vibration by a train are absorbed by its elasticity. As a railroad of a super-express train, a slab track where the sleepers and the ballast are replaced by concrete and mortar is used for alleviating track maintenance. There are three roles of the sleeper, that is, a sleeper function for alleviating a load applied on the bed by distributing the weight of train widely over the bed and preventing rails from causing aged sinking, a tie function of maintaining constant an interval (rail gauge) between left and right rails and a lateral resistant and longitudinal resistant function to make rails immovable in the left and right direction and in the forward and rearward direction against the operation of a lateral pressure and a force in the direction of the axes of the rails.
It is said that the total number of sleepers used in railroads in the world amounts to three billion pieces, among which four hundred million pieces are sleepers made of concrete. Sleepers made of wood currently share an overwhelmingly high proportion as indicated by the Chinese ideograph for "pillow wood". However, the use of sleepers made of wood which gives rise to exhaustion of forest resources tends to be restricted year by year in view of environmental conservation, among all preservation of forest resources. Therefore, an enormous worldwide demand is estimated in the future for sleepers made of concrete so as to contribute to environmental conservation, and a further expectation is put therefor. Approximately twenty million pieces per year of sleepers made of concrete are produced as a substitute for sleepers made of wood, as well as sleepers for new line construction. Especially in advanced countries with railroads such as Europe, Japan and the like, approximately one half the yearly demand for sleepers includes sleepers made of concrete.
As illustrated by FIG. 3, according to a sleeper track of a conventional railroad, sleepers 32 are arranged orthogonally to the axial direction of two rails 31a and 31b at predetermined intervals (for example, about 750 mm). A sleeper made of wood or a sleeper made of concrete is used for the sleeper 32. According to the sleeper track, the sleepers constitute bases placed independently from each other for intermittently supporting the rails and accordingly, they support rails only partially. Therefore, every time a train passes through portions of the rails which are not brought into contact with the sleepers vibration occurs and a noise referred to as "rolling sound" is caused. Further, in respect of a track where a ballast is laid on a bed, warp of the rails is liable to occur by repeated application of the weight of the train, which gives rise to rocking of a running train. An increase in the rocking of a running train accelerates deterioration of the track.
Meanwhile, the formation of the sleepers made of concrete has progressed along with long welded rails a means for acceleration and comfortability of transportation. A large temperature dependent axial compression force (a force stored at the inside of rails for fixing unmovably the rails by sleepers against elongation and contraction of the rails by the change in temperature) is operated on the rails with the formation of long welded rails and therefore, rails are bent in the upper and down direction and in the left and right direction when the fixing force is weak. In order to prevent the deformation of the rails, sleepers made of concrete which have a large weight and a large rail fastening force becomes an important track component factor exceeding a role of a substitute for sleepers made of wood in the present age high speed railroad.
Although as described above sleepers made of concrete constitute an important track component factor exceeding the role of the substitute for sleepers made of wood in the present age high speed railroad, the configuration thereof is quite equal to that of the sleepers made of wood whereby achievement of labor saving in maintenance of ballasted tracks is limited. Especially, enormous cost and labor are necessary in the maintenance of ballasted tracks, however, the maintenance operation is referred to as a representative of the so called "3K operations" in Japan (dangerous, laborious and dirty operation) as a result of limiting the operational maintenance time to the middle of the night with respect to the operational time of the railroad and is an operation avoid by young people.
Recently, a ladder-type sleeper was disclosed in "RRR", 1995. 12, p8-28, issued by a foundation of the Railway Technical Research Institute for reducing the maintenance on ballasted track. Specific structure and feature of the ladder-type sleepers for railway tracks will be explained in reference to FIG. 4 and FIG. 5. In respect of longitudinal beams 41 bearing the sleeper function, there is adopted a pretension type prestressed reinforced concrete structure (hereinafter, referred to as a pretension type PRC structure) where upper and lower prestressing strands 42 are pretensioned and the tension force is released after curing a fed concrete 43 and the longitudinal beams 41 having the pretension type PRC structure are designed by a limited design method whereby a necessary and sufficient load bearing function can be ensured even with a least sectional area under conditions of a ballasted bed where a disadvantageous support state must be predicted. With respect to connectors bearing the tie function, small diameter thick wall steel pipes 44 having a rigidity necessary for holding the rail gauge, are arranged at intervals of 3.0 m, embedded portions 45 of the small diameter thick wall steel pipes 44 are inserted between the upper and lower prestressing strands 42 which constitute main axial reinforcement members of the longitudinal beams 41 and which are under tension and a mixed rigid connection structure having a ladder-like shape is formed by integrating the connectors solidly with the longitudinal beams 41 by feeding concrete 43 whereby the tie function is sufficiently ensured. Incidentally, numeral 46 in the drawings designates a star lap, numeral 47 designates an embedding metal piece, numeral 48 designates a rail and numeral 49 designates a rail fastener.
With respect to the small diameter thick wall steel pipe bearing the tie function of the ladder-type sleeper for railway tracks, a carbon steel pipe for general structure STK 540 prescribed in JIS (Japanese Industrial Standards) G3444 is used in view both of rigidity and operational performance and in view of cost and ribs 51 are fixedly welded to both end portions of the small diameter thick wall steel pipe 44 as shown by FIG. 6. Also, portions of the small diameter thick wall steel pipe 44 exposed to the atmosphere are coated with a rubber lining 52 by a curing treatment for preventing corrosion since waterproof, stray current resistance (insulation performance), impact resistance and weather resistance are necessary for these portions in view of environments where they are used.
With respect to the installation of the ribs onto the small diameter thick wall steel pipe bearing the tie function of the ladder-type sleeper for railway tracks, working steps of rib working, rib welding and the like are necessary, which poses a significant problem for the ladder-type sleepers for railway tracks in view of a further reduction in cost and achievement of mass production. Also, the execution of the rubber lining by a curing treatment for preventing corrosion poses a problem of necessitating the curing treatment cost since the small diameter thick wall steel pipes are needed to transport to a rubber lining executing company where a baking is executed.